Classroom activity Grade 8 Engineering & Design

Cloverbank's Cloudy River: A Coagulation Jar-Test Design Challenge

A grade 8 engineering activity where teams design a coagulant recipe on a points budget, test it on muddy water, and iterate to reach the clearest cup.

Grade: Grade 8
Subject: Engineering & Design
Material: Classroom activity
Time: 50 min
Prep: Moderate prep
Materials: Muddy water made from soil stirred into tap water (about 2 liters per group), Clear plastic cups (4 per group), Alum (aluminum sulfate) solution with dropper, Stir sticks, Timer or clock, Turbidity comparison card with a 1-to-5 cloudiness scale, Points-budget cards, Safety goggles for each student

Standards

MS-ETS1-3 NGSS

Analyze data from tests to determine similarities and differences among several design solutions to identify the best characteristics of each that can be combined into a new solution to better meet the criteria for success.

How this material aligns: Teams test a coagulant recipe, compare clarity results across groups, and use those results to improve a second design, the analyze-and-compare testing this standard requires.

All materials for MS-ETS1-3 →

MS-ETS1-4 NGSS

Develop a model to generate data for iterative testing and modification of a proposed object, tool, or process such that an optimal design can be achieved.

How this material aligns: The jar test is a physical model teams run to generate clarity data and iterate their coagulant dose, the model-and-iterate work this standard describes.

All materials for MS-ETS1-4 →

Download Word file

This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-23.

Objectives

Students will design a coagulation recipe within a points budget and test it on muddy water.
Students will use clarity results to improve a second design and explain why the change helped.

Vocabulary

Coagulation: Coagulation is the first cleaning step at a drinking water plant. Operators quickly mix in a chemical that makes tiny bits of dirt stop pushing each other away so they can start sticking together.
Floc: Floc is the fluffy clumps of dirt and treatment chemicals that form in the water during cleaning at a treatment plant. Floc looks like tiny snowflakes drifting in the water.
Turbidity: Turbidity is a measure of how cloudy or murky water looks because of tiny particles floating in it. Clear water has low turbidity, and muddy water has high turbidity.

Also: dose, settling

Teacher overview

This is a small-group design challenge for teams of three or four. Cloverbank's river is a fictional muddy river, and teams act as treatment engineers who must clear the water. Real plants add a coagulant that makes tiny mud particles clump into larger clumps called floc, which then settle out. Each team designs a recipe (how much coagulant and how long to mix and settle) while spending from a fixed points budget, tests it, rates the clarity, and then gets one redesign round. The points budget maps onto a real plant's cost limit, because chemicals and mixing time both cost money, so the goal is the clearest water for the fewest points.

Activity Directions

Setup (8 min). Give each team about 2 liters of muddy water, four clear cups, a dropper of alum solution, a stir stick, a turbidity card, and a points-budget card worth 10 points. Explain the price list: each drop of coagulant costs 1 point, each 30 seconds of stirring costs 1 point, and settling time is free. Safety: the alum and muddy water are for testing only. Everyone wears goggles while handling the alum solution. No tasting or drinking, and everyone washes their hands after.
Rate the start (4 min). Each team pours one cup of muddy water and rates its cloudiness on the 1-to-5 card, where 1 is clear and 5 is very cloudy. They record this starting rating.
Design round 1 (8 min). Teams plan a recipe on paper: number of coagulant drops, seconds of stirring, and minutes of settling, keeping the total points at or under 10. They write down the plan before touching the water.
Test round 1 (8 min). Teams add the drops, stir for their planned time, then let the cup sit still to settle. When settling time ends, they rate the clarity on the same 1-to-5 card and record it.
Compare (4 min). Teams post their starting rating, ending rating, and points spent where everyone can see. As a class, look for which recipes cleared the most cloudiness per point.
Design round 2 (8 min). Using what other teams found, each team designs a second recipe, still at or under 10 points, aiming to beat their round 1 result. They test it in a fresh cup and rate the clarity.
End condition (2 min). The challenge ends after both test rounds. The winning team has the clearest ending rating while staying within the points budget.
Debrief (8 min). Ask the debrief questions below and have teams explain the single change that helped their water most between round 1 and round 2.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

Debrief question: Why does adding a coagulant make the water clearer? Answer: The coagulant makes tiny mud particles stick together into larger, heavier clumps called floc, which sink faster than the tiny particles could on their own, so more mud settles out and the water looks clearer.
Debrief question: What happened when a team added too little coagulant? Too much? Answer: With too little coagulant, few particles clumped and the water stayed cloudy. With too much, teams often wasted points without much added benefit, and some found the water no clearer than a smaller dose, which shows there is a best dose rather than "more is always better."
Debrief question: Why does settling time matter? Answer: Floc needs still time to sink to the bottom. If the cup is rated too soon or is stirred while settling, the clumps stay mixed in and the water still reads cloudy, so settling time is part of the design.
Debrief question: How is the points budget like a real plant's budget? Answer: Real plants pay for chemicals and for the energy to mix, so operators use the smallest dose that still works. The points budget pushes teams to find the clearest water for the fewest points, the same trade-off a plant makes.
Debrief question: Real plants must reach 0.3 NTU. Could your cup meet that? Answer: No. A jar test only settles out the larger, visible mud. Reaching 0.3 NTU takes filtration and continuous testing after coagulation, so the classroom cup is a simplified model, not finished drinking water.

Differentiation

Support

Give these teams a starter recipe (for example, three drops, one minute of stirring, three minutes of settling) as a first design so they begin with a working baseline and adjust from there. Pair a reader with a recorder in each team.

Extension

Ask teams to run a controlled test that changes only one variable between two cups, such as stirring time, while keeping drops and settling equal, then write a short claim about how that one variable changed the clarity.

ELL supports

Provide picture steps for the procedure (drop, stir, settle, rate) and a turbidity card that uses shaded circles for each 1-to-5 level. Pre-teach coagulant, floc, and settle with a short demonstration before teams begin.

Extensions

Have the class write a one-page recommendation to a fictional Cloverbank water board naming the recipe that cleared the most cloudiness per point, with a labeled sketch of the steps. This turns the test data into an engineering recommendation with an audience.